This invention relates to a laser doppler velocity simulator and in particular to a laser doppler velocity simulator in which a laser beam is passed through a rotating refractive transparent block whereby the beam optical path length is continuously changing thus inducing a doppler frequency shift in the laser beam.
A variety of anemometers are in use for measuring the magnitude of fluid low velocity in a wind tunnel. Among these are pitot tubes, hot wires, and reference beam laser velocimeters. The reference beam laser velocimeter measures the doppler frequency shift induced in a reference beam reflected from a moving target object, such as a dust particle or smoke particle suspended in a fluid stream. The magnitude of doppler shift is indicative of the velocity of the target object and the fluid. This device is especially useful where fine resolution is desired as in fluid boundary layers adjacent to model surfaces in wind tunnel tests.
In testing and evaluating methods for directional ambiguity removal for a reference beam laser velocimeters, a need arose for a velocity simulator to simulate motion in the direction of the illuminating beam and reversible in direction periodically. Furthermore, it was desired that the returned beam be collinear with the incident beam and that doppler shifts of 5 megahertz or more result. Several schemes including ground discs and speaker mounted mirrors were investigated but proved inadequate.
The present invention meets the above criteria and provides a new approach for testing and calibration of reference beam laser velocimeters by simulating the movement of a target object. A rotating glass block continuously changes the optical path length of the laser volocimeter reference beam thus inducing a doppler frequency shift therein. The magnitude of the frequency shift is a function of the known rate of change of the optical path length. The doppler frequency shift registered by the velocimeter is compared to the calculated true doppler frequency shift for calibration.
An object of the present invention is a method and apparatus to induce a doppler frequency shift of known magnitude in a laser light beam.
A further object of the invention is a method and apparatus to induce a doppler frequency shift in a reference beam laser velocimeter laser beam to simulate target velocity having a component in the direction of the beam and reversible in direction periodically.
A further object of the invention is a laser doppler velocimeter which yields the foregoing advantages and in which the return beam is collinear with the incident beam and which produces doppler shift of at least 5 megahertz.
Other objects and advantages of the present invention will be readily apparent from the following description and drawings which illustrate preferred embodiments of the present invention.